1. Field of the invention:
This invention relates to a method for coating a metal substrate by the use of a resin composition comprising a major amount of polyvinylidene fluoride. More particularly, it relates to a method for preparing a coated metal substrate with polyvinylidene fluoride with excellent hot-water resistance and chemical resistance at high temperatures.
2. Description of the prior art:
Polyvinylidene fluoride (PVdF) has excellent mechanical properties, heat resistance, and weather resistance. Also, it has excellent water resistance and chemical resistance. Therefore, this kind of resin is used as a coating material for films that are used for packaging chemicals, as a coating material for containers in which corrosive chemicals are stored, or as a coating material for protecting metal parts used in various factories in the chemical and food industries. As mentioned above, PVdF has many excellent characteristics. However, because PVdF has a high degree of crystallinity, residual stress arising from the crystallization of PVdF will easily form in the coating layer containing the PVdF. Moreover, like other fluororesins, PVdF has poor adhesion to metals and to other resins. Therefore, when a substrate coated with PVdF is immersed in hot water or exposed to chemicals at high temperatures for a long time, the coating layer can crack or peel because of the residual stress, and can blister and peel because of the permeation of hot water or chemicals such as acids into the coating layer.
There have been proposed various methods of reducing the residual stress arising from crystallization of PVdF after its coating. For example, Japanese Patent Publication No. 46-2918 discloses a method for heat treatment of a coating layer made of PVdF formed on a metal substrate by conventional techniques such as electrostatic coating and baking. In this method, the coated substrate with PVdF is reheated at a given temperature to melt a small part or most of the PVdF crystal, and then cooled gradually. The given temperature mentioned above may be the temperature at which the PVdF begins to melt (i.e., a slightly lower temperature than its melting point) to the temperature higher than the melting point by 10.degree. C., and preferably from the melting point to the temperature that is higher than the melting point by 10.degree. C. Japanese Patent Publication No. 47-15233 discloses a method comprising the formation of a PVdF coating layer on a metal substrate by the powder coating technique and the like, heating and melting the PVdF coating layer, and then precooling the coating layer within a given temperature range followed by rapid quenching to a given temperature (80.degree. C.). The temperature for precooling may be in the range of from the temperature that is higher than the crystallization temperature of PVdF by 10.degree. C. to its melting point.
According to the methods disclosed in the above-mentioned publications, the residual stress of PVdF will be reduced, resulting in less cracking and peeling of the PVdF coating layer. However, the PVdF coating layer consists generally of a crystallized part with relatively high density in which the orientation of PVdF molecules is regular, and a rubber part with relatively low density in which the orientation is irregular. Thus, hot water or chemicals such as acids permeate from the rubber part into the PVdF coating layer, and the coating layer of PVdF blisters or peels.